High-pressure gas discharge lamp

ABSTRACT

A high-pressure gas discharge (HID, high intensity discharge) lamp is described, having a discharge vessel ( 1 ) that contains a metal that is applied at least to parts of those regions of the feedthroughs and/or walls of the discharge vessel ( 1 ) at which condensation of ingredients of the gas filling may occur as a result of a temperature sink that occurs when the lamp is in a state of operation. In this way, it is possible to at least largely prevent not only chemical interactions between ingredients of the gas filling (particularly the metal halides) and the relevant regions of the feedthroughs or walls at which, due to the lower temperature of these regions, the ingredients condense, but also losses of ingredients of this kind from the gas filling. This in turn means that there is no risk of any damage to the lamp or of any degradation of its lumen maintenance.

DESCRIPTION

The invention relates to a high-pressure gas discharge (HID, highintensity discharge) lamp having a discharge chamber for a gas filling.

Due to their good lighting properties, high-pressure gas discharge lampshave become widely used. They generally comprise a discharge vesselhaving feedthroughs through which electrodes extend into the dischargevessel, or rather into the discharge chamber enclosed by the latter.When the lamp is in the operating state, an arc discharge is excitedbetween the opposing free ends of the electrodes.

The discharge chamber generally contains a gas filling (lamp filling)comprising a starter gas (such as argon for example), a discharge gas(such as one or more metal halides such as sodium iodide and/or scandiumiodide for example), which forms the actual light-emitting material(light producer), and a voltage-gradient generator or buffer gas (suchas mercury) whose principal function is to promote the evaporation ofthe light-producing substances by raising the temperature or pressure,and to increase the efficacy and burning voltage of the lamp.

For the electrodes to be accurately and permanently positioned with agas-tight seal, the standard of the feedthroughs needs to meet stringentrequirements. To enable these requirements to be met, both the materialof which the feedthroughs are made (such as quartz or polycrystallineAl₂O₃ [PCA]) and the material of the electrodes (such as tungsten,molybdenum, niobium for example) are of particular importance.

In the case of the CDM lamps made from PCA, the electrodes, which aremanufactured from tungsten, molybdenum or niobium (the latter is used tomatch the coefficient of expansion of the electrode to that of the wallmaterial), are for example fixed in the associated feedthrough with aseal by means of a so-called fused glass, comprising a mixture ofhigh-temperature oxides such as, for example, Al₂O₃, Dy₂O₃ and SiO₂(“AlDySi”).

A problem that often exists in this case is that the metal halidescontained in the gas filling react with the electrodes and/or the fusedglass and sometimes penetrate into the feedthroughs and cause leaks inthem.

To minimize chemical interactions of this kind, various measures areadopted to attempt to keep the feedthroughs at a temperature that is aslow as possible. However, because the metal halides that are present ina gaseous state in the gas filling have the property of migratingtowards temperature sinks of this kind, and then of at least partlycondensing there, they are lost to the discharge gas while the lamp isworking and are no longer available for their true purpose, namely toincrease the particle concentration and the light emission in theplasma.

Something else that is observed is that, even when steps of this andother kinds are taken, the possibility still cannot be entirely ruledout, when high intensity discharge lamps are in use for long periods, ofchemical interactions taking place between the electrodes andingredients of the gas filling, particularly in the region of thefeedthroughs, and of unwanted effects thus occurring. In the case oflamp envelopes made of quartz glass, interactions of this kind takeplace particularly at the fused seals of the electrodes, and in the caseof PCA lamps in the fused glass used for sealing purposes.

These interactions or unwanted effects are in particular transportprocesses involving ingredients of the gas filling and also, in caseswhere quartz glass is used in the wall of the discharge vessel,recrystallization of the quartz. The effects may cause a clouding of thedischarge vessel, shifts in the color temperature of the light emitted,and a more marked degradation of the lumen maintenance of the lamp.

It is therefore an object of the invention to provide a high-pressuregas discharge lamp in which the chemical interactions betweeningredients of the gas filling and the electrodes, the inner wall of thedischarge chamber and the feedthroughs are at least substantiallyreduced.

The aim is also to provide a high-pressure gas discharge lamp in whichthe loss of ingredients from the gas filling, due in particular totransport processes connected with temperature sinks and/or tocondensation, is at least substantially lower while the lamp isoperating.

In accordance with claim 1, the object is achieved by a high-pressuregas discharge lamp having a discharge vessel containing a metal that isapplied at least to parts of those regions of the feedthroughs and/orwalls of the discharge vessel at which condensation of ingredients ofthe gas filling may occur due, in particular, to temperature sinks thatoccur when the lamp is in a state of operation.

One advantage of this solution is that it renders it possible at leastto minimize not only said chemical interactions between ingredients ofthe gas filling and the regions of the feedthroughs and/or wall at whichthe ingredients condense, due in particular to the lower temperature ofthese regions, but also losses of ingredients of this kind from the gasfilling. In this way, any risk either of the lamp being damaged or ofits lumen maintenance being degraded can be avoided to a considerabledegree.

The dependent claims relate to advantageous embodiments of theinvention.

The embodiment to which claim 2 relates has the advantage that the metalcan be applied to said regions relatively easily, for example, byproducing a temperature sink in said regions by heating and/or coolingthe lamp before the lamp is put into operation for the first time, onwhich regions the metal will then deposit.

The embodiments to which claims 3 and 4 relate reduce in particular thetransport of electrode material which may cause the wall of thedischarge lamp to become darkened.

The embodiment to which claim 5 relates prevents in particular chemicalinteractions between the gas filling and a fused glass used for sealingpurposes.

The metal that is introduced in accordance with the invention may alsoitself be used for sealing purposes, as claimed in claim 6, or, asclaimed in claim 7, may correct flaws in the wall of the dischargevessel and/or of the feedthrough (particularly what are called shrinkholes).

Claim 8 gives examples of the metals according to the invention.

Finally, claim 9 deals with a preferred method by which the metal can beapplied to said regions in a particularly simple and effective manner.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic longitudinal section through a high-pressuregas discharge lamp according to the invention.

The invention will be described below with reference to a CDM lamphaving a PCA wall material. The invention may, however, be applied toall other types of high-pressure gas discharge lamps, in which case thesealing materials according to the invention may vary as a function ofthe wall material and the nature and design of the feedthroughs.

FIG. 1 is a diagrammatic view of a high-pressure gas discharge lamp ofthis kind. The lamp comprises a discharge vessel 1 which encloses adischarge chamber 11. The wall 12 of the discharge vessel 1 is made ofpolycrystalline Al₂O₃ (PCA).

Extending into the discharge chamber 11 from opposite ends thereof arethe free first ends 2, 3 of electrodes, which electrodes are made of amaterial, such as tungsten, having a melting point that is as high aspossible. The other ends of the electrodes are in contact withrespective electrically conductive ribbons (or foils) 4, 5, made inparticular of molybdenum or cermet, the ribbons 4, 5 being connected inturn to respective terminal pins 6, 7 made of, for example, niobium. Thefree ends of the terminal pins 6, 7 finally form the external electricalcontacts of the discharge lamp.

To ensure a vacuum-tight entry for the electrodes into the dischargechamber 11, the discharge vessel 1 is provided with two feedthroughs 8,9 (pinches) that have embedded in them respective ones of theelectrodes, respective electrically conductive foils 4, 5, and portionsof respective terminal pins 6, 7. At their outer ends, the feedthroughs8, 9 are sealed off with seals 81, 91 made of fused glass. Typicalcomponents of this fused glass are varying proportions of Al₂O₃, Ln₂O₃(Ln=a rare earth metal), and SiO₂.

When the lamp is in the operating state, an arc discharge (alight-generating arc) is excited between the first (free) ends 2, 3 ofthe electrodes.

For this purpose, the discharge chamber 11 is filled with a gas whichcomprises not only a starter gas, such as argon for example, but also adischarge gas (light producer) which emits light radiation as a resultof excitation or discharge and, preferably, a voltage-gradient generatoror buffer gas, both of which latter gases may be selected from the groupof metal halides.

The light-producing substances are in particular mixtures of differentmetal halides such as NaI, DyI₃, HoI₃, TmI₃ and TlI (thallium iodide),while Hg or Zn or ZnI₃ can be used as a voltage-gradient generator orbuffer gas.

Some of these metal halides normally migrate to the colder regions ofthe discharge vessel 1 and in particular to the mouth regions of thefeedthroughs 8, 9 and the regions of wall surrounding them, condensethere and form a deposit 20, which may result in the degradation of lampperformance described above and in damage to the lamp.

To prevent this from happening, there is also introduced into thedischarge chamber 11 a metal which is substantially liquid, or in otherwords is present as a molten phase, at the normal temperatures of thesaid colder regions and particularly the regions of the feedthroughs 8,9. This metal is added in a quantity that is sufficient to coat thoseregions of the feedthroughs and/or walls that are at risk from thedeposit of metal halides (which regions may be referred to in general asthe temperature sinks), i.e. particularly the space between theelectrodes and the inner walls of the feedthroughs 8, 9, and theadjacent regions of the wall 12 of the discharge vessel 1. Anyhair-cracks that may exist in the wall 12 of the discharge vessel 1 andthat of the feedthroughs 8, 9 are also plugged in this case.

A particularly good way of causing the liquid metal to be transported tothese regions is to set up an appropriate temperature gradient withinthe lamp before the lamp is put into operation for the first time and,if required, at given intervals of time, so that said regions are at alower temperature and the liquid metal migrates to these regions andlines them.

A suitable temperature gradient may for example be set up by heating thelamp, in the switched-off state, from the outside in the region of thedischarge vessel 1 and/or cooling it from the outside in the region ofthe feedthroughs 8, 9.

Metals that are suitable for this purpose are, for example, aluminum,zinc, tin, bismuth and indium.

This achieves in particular that the metal entirely covers those regionsof the electrodes at which the electrodes enter the discharge vessel 1(the roots of the electrodes), i.e. the regions at which thefeedthroughs 8, 9 open into the discharge vessel 1, which regions aresealed with fused glass.

The advantages achieved in this way are, amongst others, the following:

The covering of the roots of the electrodes also substantially reduces,or stops, the transport of tungsten from the electrodes, which may reachcritical levels at the high temperatures that are usual, therebyimproving the lumen maintenance of the lamp and largely preventing thewall 12 of the discharge vessel 1 from being darkened.

The fact of the fused glass being covered by the metal prevents chemicalinteractions between the metal halides in the gas filling and the fusedglass. Exchange reactions between the metal halides andrare-earth-containing ingredients of the fused glass, which may causeincreased corrosion of the fused glass and considerable fluctuations andshifts in the color properties of the light emitted, are avoided.

The metals that are introduced also prevent the metal halides containedin the gas filling from being transported chemically as a result of thetemperature gradient from hot to cold.

The result of this is, on the one hand, that the dosage of the(corrosive) metal halides in the gas filling can be greatly reducedbecause they are not lost while the lamp is operated as a result oftheir migrating to the colder regions and condensing there. This is thusanother way in which the chemical interactions with the metal halidescan be reduced to a corresponding degree.

Because, on the other hand, the composition of the molten metal halidephase remains largely constant over the life of the lamp, the colorproperties of the light emitted are substantially more constant toothroughout the whole of the lamp's life.

If suitably chosen, the metal could also perform all or part of thefunction of the sealing material in the feedthroughs, thus enabling thefused glass to be at least partly dispensed with.

In the case of discharge lamps whose walls are made of quartz glass, theuse of the metal mentioned provides the additional advantage that whatare called shrink holes, that occasionally form during the fusing of thequartz to produce a seal and are a frequent cause of the prematurefailure of lamps, can also be plugged.

1. A high-pressure gas discharge lamp having a discharge vessel (1)containing a metal that is applied at least to parts of those regions ofthe feedthroughs and/or walls of the discharge vessel (1) at whichcondensation of ingredients of the gas filling may occur due, inparticular, to temperature sinks that occur when the lamp is in a stateof operation.
 2. A high-pressure gas discharge lamp as claimed in claim1, wherein the metal has a substantially molten phase in the temperaturerange of the temperature sink.
 3. A high-pressure gas discharge lamp asclaimed in claim 1, wherein the metal is applied at least to parts ofthe regions between a feedthrough (8; 9) and an electrode supportedtherein.
 4. A high-pressure gas discharge lamp as claimed in claim 1,wherein the metal is applied at least to parts of those regions (2, 3)of the electrodes at which the electrodes enter the discharge vessel (1)(the roots of the electrodes).
 5. A high-pressure gas discharge lamp asclaimed in claim 3, wherein the electrodes are positioned in thefeedthroughs (8, 9) with fused glass, the metal covering the fused glassin this case.
 6. A-pressure gas discharge lamp as claimed in claim 3,wherein the electrodes are positioned in the feedthroughs (8, 9) so asto be sealed by means of the metal.
 7. A high-pressure gas dischargelamp as claimed in claim 1, wherein the metal is applied to regions ofthe feedthroughs and/or walls in which shrink holes are situated, toplug the latter.
 8. A high-pressure gas discharge lamp as claimed inclaim 1, wherein the metal comprises one or more materials from thefollowing group: aluminum, zinc, tin, bismuth and indium.
 9. A method ofapplying a metal having a substantially molten phase in the temperaturerange of a temperature sink to, at least, parts of regions offeedthroughs or walls of a high-pressure gas discharge lamp, at whichparts of regions condensation of ingredients of a gas filling of adischarge vessel (1) of the high-pressure gas discharge lamp, may occurdue, in particular, to temperature sinks that occur when the lamp is ina state of operation, wherein the method comprises at least the step ofproducing at least one temperature sink in at least one of said regionsby heating and/or cooling the lamp from the outside, during which thelamp is not in operation.